Alloy steels



United. States Patent 3,027,253 ALLGY STEELS Ralph B. G. Yen, Westfield, N.J., assignor to The International Nickel Company, Inc., New York, N.Y., a corporation of Delaware No Drawing. Filed Apr. 20, 1960, Ser. No. 23,381 3 Claims. (til. 75--128) The present invention relates to alloy steels and, more particularly, to low alloy steels characterized by extremely high yield and tensile strengths in combination with satisfactory ductility and toughness, thus making the new alloy steels especially suitable for use as structural elements or components, e.g., aircraft landing gears and the like.

As is known by those skilled in the art, efforts have been intensified to develop new alloy materials capable of meeting ever increasing requirements of various industrial and commercial applications. That specification requirements will become more stringent is quite evident from recent developments in the aircraft industry where heavier aircraft are being designed to accommodate greater load carrying capacity, thus unavoidably necessitating the use of structural elements of increased strength. In recent years, it would appear that interest in alloy steels has been revived concerning applications requiring a good combination of mechanical properties including high yield and tensile strengths, ductility, toughness, etc. A general survey of the literature reflects that for many years there was considerable reluctance to employ, for example, heat treated alloy steels of high strength as structural elements primarily because of the brittleness considered to attend such steels. At one time a steel of about 200,000 pounds per square inch tensile strength was thought to be about the limit for safety purposes in various structural components. Sufiice to say, steels of such strengths no longer serve the more imposing demands created by scientific development. This aspect is exemplified by the aircraft industry where advanced designs now require structural elements possessing yield strengths of over 250,000 p.s.i. The designer of structural elements or components is limited functionally by, inter alia, the maximum yield strength and ductility properties 7 characteristic of a material. It is to these properties to which the present invention is particularly directed since they exert a predominating influence with regard to what materials a designer can employ for various structural components.

The development of alloy steels of high strength is not without difficulty, it being appreciated that high strength in and of itself is not the ultimate and only important criterion. An alloy steel might possess what was heretofore considered high yield strength, say, 240,000 p.s.i., but possess poor ductility or toughness. Too, such an alloy steel might not be amenable to necessary heat treatments. High strength alloy steels should be capable of being tempered at temperatures of at least about 600 F. to insure adequate stress relief, but many prior art steels have been inherently prone to develop brittle failure if tempered at such temperatures. Too, the application of such tempering temperatures has often resulted in a detrimental loss of strength in some prior art steels.

Alloy steels possessing a yield strength of 250,000 p.s.i. and a tensile strength of 290,000 p.s.i. are classed in the art as being high strength steels. Some authors have theorized that the maximum tensile strength that would ever be obtained from low alloy steels heat treated in a conventional manner would be about 350,000 p.s.i. This limit is met and exceeded by alloy steels of the present invention.

Although attempts were madeto overcome the foregoing difiiculties and other disadvantages, none, as far lCQ as I am aware, was entirely successful when carried into practice commercially on an industrial scale.

It has now been discovered that alloy steels of special composition can be provided which manifest in the tempered condiiton tremendously high yield and tensile strengths, e.g., yield strengths of 275,000 p.s.i. and up to 300,000 p.s.i. or above, and ultimate tensile strengths above 300,000 p.s.i. and up to 360,000 p.s.i., together with satisfactory ductility and toughness. Further, the alloy steels are amenable to tempering treatments of over 500 F., e.g., 600 F. or higher, whereby the presence of residual stresses is eliminated or greatly reduced. The outstanding properties of the alloy steels of the invention render them eminently suitable for use as structural elements or components in aircraft, e.g., landing gears.

It is an object of the present invention to provide low alloy steels in the tempered condition characterized by extremely high yield strengths, e.g., above 275,000 p.s.i., and tensile strengths, e.g., above 300,000 p.s.i.

Another object of the invention is to provide low alloy steels which possess high yield and tensile strengths in combination with satisfactory ductility and toughness.

The invention also contemplates providing low alloy steels which manifest a combination of good mechanical properties including very high strength and which can be tempered at relatively high temperatures without detri mental loss of properties or incurrence of brittle failure.

The invention further contemplates providing structural elements or components, e.g., aircraft landing gears, formed of low alloy steels having a new and improved combination of mechanical properties.

Generally speaking and in accordance with the invention, a highly advantageous combination of properties and excellent results are achieved with alloy steels of the following preferred composition: from 0.5% to about 0.65% carbon, about 0.8%, e.g., 1% to about 6% cobalt, about 0.2% to not more than 0.45% manganese, about 1.5% to about 2.2% silicon, about 1.5% to about 2.5% nickel, about 0.5 to about 2.5% chromium, about 0.2% to about 0.6% molybdenum, the balance being essentially iron. Yield strengths of over 275,000 p.s.i. and up to 300,000 p.s.i. and over can be attained and yet ductility and toughness (impact strength) are comparable to or superior than those manifested by known alloy steels of much lower strengths. It is particularly advantageous for best impact strengths that the alloy steels contain not more than 0.01% sulfur and not more than 0.01% phosphorus. A satisfactory combination of properties can also be obtained with alloy steels containing 0.4% to 0.7% carbon, about 0.75% to about 6% cobalt, about 0.2% to about 0.6% manganese, about 1.5% to about 3% silicon, up to 3% nickel, about 0.5% to 5% chrornium, about 0.2% to about 1% molybdenum, the balance being essentially iron.

In carrying the invention into practice, the alloy steels should be initially austenitized at a suificiently high temperature to dissolve all carbides. This austenitization treatment can be carried out over a temperature range of about 1650" F. to about 1800" F., a temperature of 1750" F. being quite satisfactory. The duration of this initial austenitization treatment is dependent upon the temperature employed but a period of about one hour is ordinarily suflicient. After cooling to room temperature, the steel can be suitably quenched, e.g., oil quenched, from a second austenitizing temperature of about 1600" F. to about 1750 F. with 1650 F. being satisfactory. Subsequent to quenching, the alloy steels can be tempered at temperatures up to at least 600 F. without encountering significant loss in properties. A tempering temperature of at least about 600 F. should be employed to insure adequate stress relief.

For the purpose of giving those skilled in the art a better understanding of the invention and/or a better appreciation of the advantages of the invention, there is given herein data illustrative of the markedly improved combination of properties characteristic of alloy steels within the invention. In Table I below, there is tabulated a series of alloy compositions some of which (Alloys G, H, I, J and K) are outside the invention, while alloys A through. F illustrate the. invention. The alloys were heat treated by austenitizing at about 1700" F. or 1750 F. for hour, air cooling to room temperature, austenitizing at about 1600 F. or 1650 F. for hour and then oil quenching. Various tempering treatments were employed regarding temperature and the duration thereof as is shown in connection with the data presented 1n Table II herein.

Table I O, 00, Mn, Si, Ni, Cr, Mo, Alloy perperperperperperpereent cent cent cent cent cent cent 1 Contained 0.47% copper.

In Table II herein there is given the data obtained in the tests conducted with the alloy steels of Table I.

Table II 0.2% Ultimate Oilset Tensile Elonga- Reduction Alloy Yield Strength tion, of Area, Strength (p.s.i.) Percent Percent p.s.i.)

1 Double tempered, 5 hrs. at 600 F., total 10 hrs. at 600 F.

Double tempered, 2 hrs. at 600 F., total 4 hrs. at 600 F.

Double tempered, 2 hrs. at 700 F., total 4 hrs. at 700 F.

The results given in Table II and particularly the results for alloys A, B, C and D indicate that alloy steels within the invention afiord markedly high yield strengths in combination with good ductility. It might be noted that yield strengths in excess of 300,000 p.s.i. which are characteristic of alloy steels Within the invention are as high or higher than even the ultimate tensile strengths of the best known conventionally heat treated alloy steels which are commercially available. Of course, the alloy steels Within the invention manifest, as will be seen from Table II, ultimate tensile strengths up to and above 350,000 p.s.1.

An important advantage of the present invention is that a high yield strength can be obtained utilizing conventional heat treatments. Recourse to special heat treatments and/or mechanical treatments, for example, re-

frigeration, cold rolling at sub-zero temperatures, rolling of metastable austenite, etc., are not required to achieve the markedly high properties of alloy steels within the invention. Such special heat treatments or mechanical treatments are often more expensive than conventional heat treatments and quite often involve tedious and burdensome processing techniques. Further, the ultimate shape of a particular product to be achieved in accordance with such other processes is often restricted, this being a characteristic disadvantage of, for example, cold rolling processes.

Alloy steels of the present invention are suitable for use in a wide variety of applications. They are particularly adaptable for use in applications requiring high strength to weight ratios such as structural components, e.g., aircraft landing gear and the like. Of course, the alloy steels can be used wherever applications require a combination of very high yield strength and satisfactory ductility properties.

As will be readily understood by those skilled in the art, the term balance when used to indicate the amount of iron in the alloy steels does not exclude the presence of other elements commonly present as incidental elements, e.g., deoxidizing and cleaning elements, and impurities ordinarily associated therewith in small amounts which do not adversely afiect the basic characteristics of the steels. For example, up to 0.1 aluminum can be present for deoxidizing purposes.

Although the present invention has been described in conjunction with preferred embodiments, it is to be understood that modifications and variations may be resorted to without departing from the spirit and scope of the invention, as those skilled in the art will readily understand. Such modifications and variations are considered to be within the purview and scope of the invention and appended claims.

I claim:

1. An alloy steel characterizedby a high yield strength in the tempered condition and possessing satisfactory ductility which consists essentially of from'0.5% to about 0.65% carbon, about 0.8% to about 6% cobalt, about 0.2% to not more than 0.45% manganese, about 1.5% to about 2.2% silicon, about 1.5 to about 2.5 nickel, about 0.5% to about 2.5% chromium, about 0.2% to about 0.6% molybdenum, and the balance essentially iron.

2. An alloy steel characterized by a high yield strength in the tempered condition and possessing satisfactory ductility which consists essentially of from 0.5% to about 0.65% carbon, about 0.8% to about 6% cobalt, about 0.2% to not more than 0.45% manganese, about 1.5% to about 2.2% silicon, about 1.5% to about 2.5 nickel, about 0.5% to about 2.5% chromium, about 0.2% to about 0.5% molybdenum, not more than 0.01% sulfur, not more than 0.01% phosphorus, and the balance essentially iron.

3. As a new article of manufacture, an aircraft landing gear element formed of an alloy steel consisting essentially of from 0.5% to about 0.65% carbon, about 0.8% to about 6% cobalt, about 0.2% to not more than 0.45% manganese, about 1.5% to about 2.2% silicon, about 1.5% to about 2.5% nickel, about 0.5% to about 2.5% chromium, about 0.2% to about 0.6% molybdenum, and the balance essentially iron.

References Cited in the file of this patent UNITED STATESPATENTS 1,496,979 Corning et al. June 10,1924 

1. AN ALLOY STEEL CHARACTERIZED BY A HIGH YIELD STRENGTH IN THE TEMPERED CONDITION AND POSSESSING SATISFACTORY DUCTILITY WHICH CONSISTS ESSENTIALLY OF FROM 0.5% TO ABOUT 0.65% CARBON, ABOUT 0.8% TO ABOUT 6% COBALT, ABOUT 0.2% TO NOT MORE THAN 0.45% MANGANESE, ABOUT 1.5% TO ABOUT 2.2% SILICON, ABOUT 1.5% TO ABOUT 2.5% NICKEL, ABOUT 0.5% TO ABOUT 2.5% CHROMIUM, ABOUT 0.2% TO ABOUT 0.6% MOLYBDENUM, AND THE BALANCE ESSENTIALLY IRON. 